CT imaging is extensively used for medical imaging and the imaging of objects. In CT imaging, X-rays are projected through the object being imaged, and these X-rays are detected by arrays of detectors. The X-rays are projected through the object in many different directions. The combination of X-ray trajectories through the object provides data from which the internal structure of the object can be determined. Contrast in CT images is provided by variations in X-ray attenuation within the object. No other contrast parameters are available. Therefore, accurate measurements of X-ray attenuation are required for high quality CT images.
Quantitative computed tomography (QCT) is a technique that allows for quantitative measurements of physical properties related to X-ray attenuation. QCT has been used for in-vivo quantitative measurements of bone density, for example. The uses of QCT include assessment of spinal trabecular bone, evaluation of drug therapy in the treatment of osteoporosis, screening for osteoporosis, fracture risk assessment and many others. Although QCT is now an established tool for bone densitometry, there exist major issues affecting the accuracy and precision of QCT measurements.
QCT measurements and CT images are affected by beam hardening error in X-ray attenuation measurements. Beam hardening error is caused by the energy-dependence of X-ray transmission within an object being imaged. In any material, low-energy X-rays are attenuated more strongly than high-energy X-rays. Therefore, as a polychromatic X-ray beam passes through an object, the proportion of high energy X-rays in the beam increases, and attenuation decreases. Long path lengths through an object therefore appear to have an excessively small attenuation. When an image is computed, the center of an object appears to have a lower attenuation than the outer regions of the object. In this way, beam hardening error produces inaccurate measurements in QCT. Correction of beam hardening errors has been an active area of research since 1975. Some popular current correction techniques for beam hardening require strict assumptions about the X-ray attenuation characteristics of the materials within the object. The two most commonly used techniques are the water and bone corrections which assume that the materials in the scan field are either water-equivalent or dense bone-equivalent in X-ray attenuation characteristics. For more information regarding these techniques, reference can be made to "A Method for Correcting Bone Induced Artifacts in Computed Tomography Scanners" by P. M. Joseph and R. D. Spital in the Journal of Computer Assisted Tomography vol. 2, pp. 481-487, 1978 and "Post-Reconstruction Method for Beam Hardening in Computed Tomography" by O. Nalcioglu and R. Y. Lou in Physics in Medicine and Biology, vol 24, pp.330-340, 1979.
In another beam hardening correction method, calibration tubes having a known transmission characteristic are used. However, this method is often not effective for two reasons: 1) different regions in the scan field experience different degrees of beam hardening, and 2) calibration tubes cannot capture the beam hardening characteristics in vicinity of a patients bone because the calibration tube must be placed outside the body.
There exists a need in the art of CT imaging and QCT for an improved method of beam hardening correction. An improved beam hardening correction technique will provide QCT measurements and CT images with improved accuracy.